When ray of light passes through the centre of curvature of the spherical mirror it strikes along the normal that means it is incident on mirror at 900. Hence the incident ray coincides with the normal and retraces its path. Therefore angle of incidence = 0 so, According to law of reflection;
Angle of reflection = 0, hence the angle of reflection to become zero degrees, thus ray of light retraces its path.
The figure below illustrates the above situation.